Optimize animation handling.

* Drop unnecessary check that the image src is what the sprite definition says, which it never is because Image.src becomes an absolute URI, and this caused the sprite to be loaded over and over.
* Don't call window.requestAnimationFrame unless there's a custom pose with animations.
* Stop calling window.requestAnimationFrame when all animations are done.
* Call window.cancelAnimationFrame when clearing the pose and an animation is ongoing.